Recent studies suggest that taxol may be an effective agent in the treatment of ovarian cancer and other tumors. These studies further suggest that taxol may be even more effective than any currently available treatment in that anti-tumor responses have also been shown in patients with cis-platinum resistant ovarian cancer (See, e.g. McGuire, W. et al, (1989) "Taxol: unique antineoplastic agent with significant activity in advanced ovarian epithelial neoplasms." Ann. Intern. Med. 111:273-279, and Rowinsky, E. et al, (1990) "Taxol: a novel investigational antimicrotuble agent." J. National Cancer Institute 82:1247-1259). Preliminary evidence also suggests that taxol is useful in the treatment of other neoplasms (See, Rowinsky supra). As a result of these findings, taxol or a drug which has a similar mode of action could have a significant and beneficial impact on cancer chemotherapy.
While the preliminary clinical results have been extremely encouraging, there are still many problems with using taxol as a therapeutic agent. At present, the most critical problem is the lack of an inexpensive and readily available source of taxol. According to published data, taxol has only been isolated from the bark or needles of Taxus brevifolia. These plants are extremely slow growing and the collection of bark or needles and subsequent extraction from large quantities of biological material is very labor intensive. In addition, taxol has intrinsic problems of extreme insolubility. Clinical trials suggest accompanying toxic side effects of treatment leading to neutropenia and to a lesser degree, neurotoxicity (See e.g. Rowinsky, supra). Thus while taxol is a novel chemotherapeutic of unique biological activity, research leading to alternate sources of taxol or taxol analogues would be extremely useful.
Taxol has a unique mode of action. It acts to stabilize microtubules both in vitro and in intact cells. (See Schiff, P., et al, (1979) "Promotion of microtubles assembly in vitro by taxol." Nature 227:665-667; and Schiff, P and Horwitz, S. (1980) "Taxol stabilizes microtubules in mouse fibroblast cells" Proc. Natl. Acad. Sci. USA 77:1561-1565.) While there are many known agents which stabilize or promote microtubule formation in in vitro polymerization assays, there is little evidence that any of these, aside from microtubule associated proteins and divalent cations, will stabilize microtubules in intact living cells.
Since the efficacy of taxol is presumably based on its unique mode of action of stabilizing microtubules, it seems worthwhile to look for other agents that might stabilize microtubules. To date there are no other known agents that act like taxol. While it is certainly possible that taxol is truly unique in its mode of action, it is also possible that there are other molecules, either closely related to or of distinctly different molecular structure, that might work like taxol, in terms of its ability to stabilize microtubules.
Although successful taxol synthesis appears to be possible using precursors found in abundance in the leaves of the Taxus tree, such synthesis would probably not be sufficient to meet the needs of cancer treatment each year. This has led to an intensive search for alternate sources of taxol or taxol-like compounds which stabilize microtubules. The presence of taxol or taxol-like compounds would be easier to detect if a screen were available which would rescue only such compounds. The present invention provides such a screen.
The present invention discloses the use of taxol dependent mutant cells derived from CHO cells as a screen for taxol or taxol-like compounds. This screen is based on the use of cells that die in the absence of taxol because their microtubules are intrinsically unstable due to mutations. There are several apparent advantages to such a screen. It is much more selective than a general screen for cytotoxic compounds. Another advantage of this screen, as opposed to in vitro assays, is that it is selective for only those molecules that have biological activity in intact cells. Therefore the screen of the present invention would exclude all agents that might have activity in an in vitro polymerization assay but not in an intact, viable cell. Cytotoxics without taxol-like activity would not be positively selected in this screen since they would not allow for growth of the taxol-dependent cells.